The invention is based on a method for brake pressure distribution between the axles of a vehicle with a pressure medium brake. The vehicle includes rotational speed sensors which are assigned to brakeable wheels and serve as actual value sensors. The brake pressure distribution between the axles is controlled in accordance with a difference in wheel speeds between the axles or in wheel slip values between the axles at the axles. The upward exceedance of a predefined limiting value by the difference in wheel speeds between the axles or the wheel slip values between the axles during a braking operation is used to control the brake pressure distribution.
When braking, changes in dynamic axle load occur owing to the deceleration of the mass of the vehicle. In the process, the front axle is additionally loaded and the rear axle is relieved of loading with the same ratio. In short vehicles with a center of gravity which is high and/or far forward, i.e. if an unfavorable ratio of the height of the center of gravity with respect to the wheel base is present, as is the case, for example, in the field of utility or commercial vehicles in the solo operating mode of tractor vehicles of a traction vehicle/semitrailer combination, this can lead, on the one hand, to the vehicle tilting over forward, i.e., to the vehicle performing a headstand. On the other hand, the lateral guidance of the rear wheels decreases, which is critical, in particular, during a braking process around a curver or bend in the road.
In order to counteract these problems, for example, DE 27 55 156 A discloses a load-dependent braking force limiter. The braking force limiter becomes active during relatively strong braking operations in which, owing to the changes in axle load which occur, the rear axle would lock. The braking force limiter prevents a further rise in the pressure from a set value, at least in the rear axle brake cylinders. Since the load is measured but not the height of the center of gravity which is essential for the critical tendency to tilt and the relieving of loading of the rear axle, this method is too imprecise. Furthermore, permanently programmed limiting values or else axle-load-dependent characteristic curves for the brake pressure always relate to utility vehicles with certain equipment and a certain vehicle body. However, if these parameters are changed, for example, in the course of retrofitting of the utility vehicle, the defined pressure limiting values or axle-load dependent characteristic curves no longer correspond to the current conditions.
In order to improve the braking stability, DE 40 07 360 A1 discloses a generic differential slip control system in which the brake pressure distribution between the axles is controlled in accordance with an obtained difference in wheel speeds between the axles at the axles, and the upward transgression of a predefined limiting value by the obtained difference in wheel speeds between the axles during a braking operation is used to control the brake pressure distribution.
The present invention is based on the object of developing a method such that a criterion for unstable driving states and braking states is acquired in the simplest possible way, as a basis for suitable counter measures.
According to the invention, in a differential slip control system, a critical deceleration value at which the brake pressure at the rear axle is lower than a predefined brake pressure limiting value is acquired by extrapolation or by interpolation on the basis of the dependence between at least the brake pressure at the rear axle and the deceleration of the vehicle after at least two preceding braking operations.
In such a differential slip control system in which the differential slip between the front axle and the rear axle is, for example, adjusted to zero, approximately linear characteristic curves, for example the characteristic curve for the brake pressure p at the front axle VA and the brake pressure p at the rear axle HA as shown in FIG. 1 are obtained for the brake pressures p as a function of the deceleration z. Since the two characteristic curves are straight lines, to acquire the profile of these straight lines it is sufficient to acquire two reference points composed of, in each case, one deceleration value z and one brake pressure value p which is assigned thereto and to acquire the further values of the linear brake pressure deceleration relationship by interpolation or by extrapolation. The method has the advantage that with each braking operation more of such reference points are acquired and the profile of the characteristic curve for the brake pressure/deceleration can, as a result, be determined even more precisely.
In order to acquire the critical deceleration value at which the rear axle is relieved of loading in such a way that an unstable driving state would arise, that is to say at the point where, according to the differential slip control system, the brake pressure at the rear axle would have to be lower than a predefined brake pressure limiting value, the point of intersection of the straight lines which represent the brake pressure profile at the rear axle with a horizontal line which represents this brake pressure limiting value is determined. The value of the deceleration z at this point of intersection is then the critical deceleration value.
If the brake pressure limiting value at the rear axle is, for example, equal to zero, that is to say according to the differential pressure control system brake pressure should no longer be applied to the brake cylinders of the rear axle owing to relieving of loading on the rear axle during braking, the point of intersection with the axis on which the deceleration z is plotted is determined in the characteristic curve.
Alternatively, for example, the application pressure or release pressure of the brake calipers or brake linings against or from a brake disk or brake drum of the rear axle brake can also be used as a brake pressure limiting value. This is because the application or release brake pressure is very low and is just sufficient still to generate no braking force or a very low braking force. As a result, the braking forces at the rear wheels can be set in accordance with the brake pressure limiting value as a function of the loading and the height of the center of gravity in such a way that the highest possible degree of braking is achieved on the premise that there is lateral guidance of the rear wheels which is sufficient for a stable driving behavior.
In order to avoid unstable driving states and braking states, the maximum deceleration of the vehicle can then be limited, for example, to a value which is smaller than the critical deceleration value.
Alternatively, the brake pressure at the rear axle could be set to smaller values than those predefined after the differential slip control process even before the maximum deceleration has been reached. This has the advantage that the lateral guidance of the rear axle is improved further. Since the front axle is still far from its locking limit owing to the high dynamic axle load displacement, this leads to a more neutral driving behavior.
The method is defined by its high level of simplicity, and consequently only rotational speed sensors, which are present in any case within the scope of antilock systems, and a brake control device, which is likewise present in any case, are necessary in which the routines of the differential slip control process proceed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.